a. Technical Field
The present invention pertains to electrical power splitters or combiners. More particularly, the invention pertains to splitters or combiners which operate to split or combine 2:1 bandwidth RF signals. Because a splitter of the present invention may be operated as a combiner, in which case the splitter outputs become combiner inputs and the splitter input becomes a combiner output, when the following discussion refers to a splitter of the present invention the discussion also implicitly refers to a combiner.
b. Problems in the Art
Electrical signals often must be divided and/or combined. For example, the power output requirements for an RF signal ma exceed the capability of readily available RF amplifiers; and to produce the required power output, the RF signal is divided and delivered to multiple amplifiers. The amplifiers' outputs are then combined to provide a power output which none of the amplifiers could have produced individually.
Quadrature hybrid couplers (couplers) are often used in power splitters. The couplers have two inputs and two outputs, one of which inputs is connected to a termination resistance matched to the system characteristic impedance (typically 50 ohms for RF signal applications). By terminating this input in this fashion, reflections at the other input are eliminated and a one-to-one VSWR is maintained. Applying a signal to the other input of the coupler produces signals at the two outputs of the coupler, each of which contains approximately half the power from the input signal.
At one output, the 0.degree. or AC-coupled output, the phase relative to the input signal is 0.degree. and at the other output, the -90.degree. or DC-coupled output, the relative phase is 90.degree. , which is inherently characteristic of a 3 db 90.degree. coupler. Additionally, although nominally half the power is delivered to each output, the amplitude response of the coupler varies according to the frequency of the input. That is, the outputs do not each have exactly one-half the power of the input signal. The frequency-dependent amplitude characteristics of the coupler outputs are illustrated in FIG. 3 of the appended drawings. Notice that one output contains more than half the input signal and the other output, complementarily, less than half the input over a frequency range of operation. This imbalance at the outputs is typically no greater than .+-.0.4 db at most. The 0.degree. output will typically have a maximum amplitude output of -2.6 db located at the center frequency, and the -90.degree. output normally has a minimum amplitude output of -3.4 db at the center frequency.
When one coupler drives two other couplers, thereby creating a four-way power divider, the imbalance at the four outputs of the two driven couplers is typically .+-.0.8 db. Two of the outputs are normally balanced at approximately -6.0 db (1/4 the input power), but the other two outputs are unbalanced. One output is typically at -5.2 db and the other output is at -6.8 db (in contrast to the nominal, desired -6.0 db).
If these divided signals were then sent to four amplifiers for amplification, one of the amplifiers would be presented a signal at approximately -5.2 db; that is approximately 30 % of the input signal power, rather than the desired 25 %. Because it is best to use identical amplifiers, each amplifier would have to be sized to handle 30 % of the input signal value. This requirement obviously limits amplifier selection, requires greater amplifier capacity, and reduces reliability due to the fact that one amplifier is carrying an excess burden that should, ideally, be shared among four amplifiers. This amplitude imbalance and its concomitant demands on amplifier capacity, reduced selection, and reduced reliability is the major shortcoming of prior art splitters.
Adding a fourth coupler (see FIG. 4), balances the two unbalanced outputs, thereby solving the amplitude imbalance problem of the prior art four-way splitter. Unfortunately, there are phase errors associated with this solution which, until the present, have never been addressed. These phase errors contribute to amplitude errors which are significant enough to negate the amplitude enhancement when the four amplified signals are recombined.
It is therefore an object of the present invention to provide a new and improved splitter which exhibits no amplitude imbalance at the output of the splitters, while, at the same time, eliminating phase errors which have heretofore cancelled the beneficial effects of a four-coupler splitter. These and other objects, features, and advantages of the present invention will become apparent from the specification and claims.